This invention concerns cadmium sulfide photovoltaic cells and methods of fabricating them. In particular, this invention concerns an improved cadmium sulfide photovoltaic cell which is characterized by a bilayer of cadmium sulfide. The bilayer is formed by depositing the two cadmium-containing layers at a critical difference in temperature.
Cadmium sulfide photovoltaic cells are well known. A great deal of research effort has been expended on their development and improvement. U.S. Pat. No. 2,820,841 granted to Carlson et al. on Jan. 21, 1958 describes a typical cadmium sulfide photovoltaic cell. In general, the cadmium sulfide cells comprise a layer of polycrystalline cadmium sulfide and a photovoltaic barrier layer of a Group IB metal chalcogenide in physical contact along a substantial interfacial area. The cell further comprises conducting and collecting electrodes associated with each of the layers. The interfacial contact between the cadmium sulfide layer and the barrier layer, usually copper sulfide, operates as a photovoltaic junction. It is believed that this junction is of the P-N type and that the mechanism of photovoltaic generation involves the formation of electron-hole pairs in the cadmium sulfide layer in response to the action of incident radiation having absorbable wavelengths. The charge carriers diffuse across the junction, creating a potential difference which in turn causes an electrical current to flow in an external circuit. The energy conversion efficiency of the cell depends among other things on the electrical and optical characteristics of the cadmium sulfide layer and the barrier layer.
U.S. Pat. No. 2,820,841 teaches that the resistivity of the cadmium sulfide layer can be reduced, thereby increasing the efficiency of the cell, by contaminating or "doping" the cadmium sulfide layer with indium or gallium, which darkens the layer, making it more light absorbent in the red end of the spectrum. Preferably the doping agent or dopant is applied as a film on the cadmium sulfide layer. When the cell is heat treated to activate the junction the dopant and cadmium sulfide interdiffuse. Precise control of the duration and temperature of the activation step is crucial to avoid destruction of the junction by interdiffusion of the barrier layer and the cadmium sulfide layer or by diffusion of too much of the dopant.
U.S. Pat. No. 3,880,633 granted to Jordan et al. on Apr. 29, 1975 teaches a method of continuously fabricating photovoltaic cells, particularly cadmium sulfide photovoltaic cells. According to this method, a tin oxide layer is applied to glass as the conducting electrode. After the electrode is formed, a solution of cadmium chloride and N,N-dimethylthiourea or thiourea in water, doped with aluminum chloride, is sprayed over the tin oxide electrode.
The methods described by U.S. Pat. Nos. 2,820,841 and 3,880,633 have improved cell efficiency. Cell efficiency, i.e., the ratio of electrical output power to solar energy input, has been improved to about 5%. However, if solar cells are to be utilized for large scale production of power, areas of such cells measured in square miles are required. On this basis the economic feasibility of large scale solar cell systems depends upon continued development of more efficient cells.